Contra Costa County Fire Captain Kent Kirby says Friday morning's explosion in Walnut Creek has turned into a criminal investigation after police say this was not a gas explosion, but from a drug lab making hash oil - a concentrated form of pot.

Around 10 a.m., fire crews responded to a two-alarm explosion at a six-unit structure at 1564 Sunnyvale Avenue in Walnut Creek, close to the Pleasant Hill BART station.

The two victims suffered serious burns and were airlifted to UC Davis Burn Center.

A third person is unaccounted for.

The fire had mostly been extinguished by 11 a.m., but Kirby said a large part of the building had collapsed.

The two victims suffered serious burns and were airlifted to UC Davis Burn Center.

Lori Meyers, a witness who works in an office building about a quarter mile away, said she felt an earthquake-like jolt. "A bunch of people thought it was an earthquake, but it was a harder and faster hit than that," she said. "To me it felt like a car hit the building."

"I heard a big explosion," said Saline Hall, a witness. She said it sounded like a bomb went off.

"I woke up to the rattle," said Niki Harrington, who lives nearby. She said her friend was screaming "Get out! Get out!"

"I looked out and saw my neighbor with burns all over his body," said Vanessa Golvich, a neighbor.

Three other units in the building were so badly damaged, they will likely be red tagged, leaving dozens homeless.

A temporary shelter has been set up at the nearby Methodist Church for those displaced by the explosion.